I would say that the sword is metal as well as magical, and thus it fails.
In Ars (thankfully!), there are no "swords of force" that are simply "solid magic" - so it has to incorporate some physical Form as well - in this case, one of Terram. And the target has a ward specifically against that up and running.
In my paradigm? Sure it is! The magic gets thru - just not the physical part of the sword, unfortunately.
"Magic" needs to Penetrate to take effect - but the sword is more than just "magic". It's also metal - and that's what gets stopped.
To my vision of how this would work (and I'm sure you'll get others), it's the opposite of the flaming sword that fails to penetrate - there, just the magic is stopped, and the sword cuts because nothing stops the mundane metal part of the equation. Here, the magic never gets a chance to penetrate, because it rides on the sword, and the sword is stopped. Even if the metal is magical, it's still "metal" equally, and I see no PeVi effect that would have cause to penetrate and cancel that part of the ward.
If the 50 penetration were trying to bring down the ward, it'd come crashing down. And a ward against flame wouldn't stop a magical sword. But this ward stops metal - and in my paradigm, that's different than stopping "terram spells of a magnitude less then or equal to the level of the ward."
It is worth noting that few, if any magi would use such a ward - bloody annoying when your silver goes flying away from you... (ward against any and all metal, remember) - not to mention that having raw vis in the form of rings, etc usually means magi likes to open for some metal getting near them. So usually, you'd find a ward against non-magical steel/iron and the like... (magical steel/iron can be warded against with the parma - it is nice to be able to wear that ring of vis on your finger...)
why should it need to be of a higher level?
The standard version of ward against wood doesn't affect magical wood (probably due to a magus carrying a staff...), and it definatly doesn't affect all other vegitable material...
Because, imo, wood and "vegetable material" are as different as metal and stone. Ward against wood would not stop a tomato, but it would stop balsa as well as ebony.
The guideline reads:Level 5: Ward a target against dirt.That, for this effect, gets bumped up by 2 mags to "against metal", and we have our basic ward. "Metal" is all metal - not just iron, not just steel, not just steel and iron and any bronze in the shape of a weapon - "metal". Lead, gold, pewter tableware and the brass buttons on your vest and that ring in your ear - away.
By countless examples in canon, when a spell gets "tricky" it adds a magnitude (or more) "for complexity".
(And I'm not sure where you're getting that bit about Ward Against Wood - in the core book it's all wood, no "magical wood" exceptions.)
(The thing is, I actually took the time to specifically re-read that very spell before posting the first response - the spell description is only like 5 lines long - and I completely missed the word "non-enchanted". Completely! Like it wasn't there.
My one ex is from Poland, Kracow specifically. Told me all about the cool dragon that used to live there. But anyway, she insists that Poland has had tomatoes and potatoes all along. She simply cannot imagine these things not growing native in her home soil.
She's the kind of girl that you eventually get tired of telling her that she is wrong, and you just give up. Tried explaining that peanuts weren't really nuts once. Very frustrating.